If I step aboard a crowded train and see that the only free space is a cramped mid-seat gap, sandwiched between two tired-looking commuters, then I will invariably choose to stand. By seizing the free spot, the unavoidable encroachment into my personal space would soon spoil any comfort that might be derived from resting my legs.

A new study suggests my amygdala could be responsible for this aversion. This is the walnut-shaped brain structure, housed deep in the temporal lobe of each hemisphere, that’s previously been associated with emotional processing, especially fear. In a new case report, Daniel Kennedy and colleagues have documented a woman, known in the clinical literature as S.M., who has damage to the amygdala on both sides of her brain, and who appears to have no sense of personal space.

When asked to indicate the interpersonal distance at which she felt most comfortable as a female experimenter walked towards her, S.M. chose a gap of 34cm – smaller than any of twenty control participants, whose average preferred distance was 64cm. Moreover, when asked to rate her comfort (from one, “perfectly comfortable”, to ten, “extremely uncomfortable”) when an experimenter stood in her face, nose-to-nose with direct eye contact, she scored the situation a “one”. It was a similar story when an accomplice of the researchers stood unnaturally close to S.M. in a situation that she couldn’t have known was part of the experiment. By contrast, the accomplice himself told researchers that he found his proximity to S.M. uncomfortable. S.M. does, however, understand the concept of personal space, and is aware that other people prefer more space than she needs.

Kennedy’s team said their finding suggests the amygdala may be involved in the strong emotional reaction that underlies personal space violations. To support their case, they scanned the brains of healthy participants and tested what happened to amygdala activation when the participants were told that a researcher was standing nearer or further away from them in the scanning lab. Crucially, when the participants were told that the researcher was nearer to them, their amygdala activation was increased.

An interesting question for future research is how a sense of personal space develops. “It is possible that the amygdala is necessary for learning the association between close distances and aversive outcomes,” the researchers said, “rather than triggering innate emotional responses to close others.”_________________________________

Further reading: A companion study published in the same issue of Nature Neuroscience and involving the same patient, S.M., suggests, contrary to prior research, that the amygdala is not needed for the rapid detection of fearful faces.